1. Field of the Invention
The present invention relates to a polyester resin for use as a molding material for various moldings such as extrusion molding, blow molding, vacuum molding and injection molding, and particularly to a polyester resin suitable for use as a heat shrinkable polyester film for various packaging materials etc. and a bottle for direct blow molding. More particularly, the present invention is concerned with a polyester resin that can be used as a molding material, such as a bottle for direct blow molding, capable of improving crystallinity and, at the same time, moldability and oxidation stability without detriment to mechanical properties of polyethylene terephthalate and can also be used as a highly shrinkable polyester film having high shrinkage in a relatively low temperature region and excellent heat shrinking properties, such as less liability to uneven shrinkage.
2. Description of the Related Art
A heat shrinkable plastic film has been used for display, protection, binding, improvement in the added value of goods, etc. in the fields of containers, fishing rods, capacitors, rod-shaped fluorescent lamps, etc. and also for multipack and contact packaging for books, notebooks, etc. At the present time, the development of various applications utilizing the shrinkability and shrinkage stress of the heat shrinkable film is anticipated in many fields other than those described above.
Resins such as polyvinyl chloride, polystyrene and polyolefin have hitherto been used as a material for a heat shrinkable plastic film. These resins, however, have had problems with heat resistance, weather resistance, chemical resistance, etc. For example, a polyvinyl chloride film is a heat shrinkable film having various shrinking properties. This film, however, is liable to experience the frequent occurrence of fish eye, and goods, wherein the above film subjected to printing is used as a packing material, have a spoiled appearance and a lower value. In order to obtain a heat shrinkable film free from fish eye, it is necessary to conduct excessive quality control, so problems occur such as a significant increase in the production cost of a film. The polyvinyl chloride is disadvantageous also in that incineration at the time of disposal results in a pollution problem, additives such as a plasticizer in polyvinyl chloride bleed out with the elapse of time to cause staining due to deposition of dust etc. and they are unsafe.
In the heat shrinkable film prepared from polystyrene, although finishing after shrinking is good, the solvent resistance is so low that a special ink should be used for printing. Further, since spontaneous shrinkage occurs even at room temperature, the film should be stored in a cold place. Further, it also had a problem with disposal. Specifically, since incineration at a high temperature is necessary, large amounts of black smoke and an offensive smell are unfavorably generated at the time of the incineration.
A polyester film is greatly expected as a material capable of solving the above-described problems, and the amount of use thereof has increased. In the conventional heat shrinkable polyester film, use was made of a copolymer polyester resin comprising a dicarboxylic acid moiety composed mainly of terephthalic acid and a diol moiety composed mainly of ethylene glycol and, copolymerized with the dicarboxylic acid moiety, isophthalic acid, sebacic acid, adipic acid or neopentyl glycol. The conventional heat shrinkable polyester film, however, had no satisfactory heat shrinking properties. In particular, uneven shrinkage is liable to occur during shrinkage. For this reason, when the film covers a container, such as a bottle, and then shrunk, there occurs a problem in that letters or patterns printed on the film are distorted and the adhesion of the film to the container is unsatisfactory.
Further, compared with the polystyrene film, the polyester film is inferior regarding shrinkability at a low temperature and therefore should be shrunk at a high temperature in order to attain the necessary degree of shrinkage, which gives rise to such problems as deformation and bottle whitening etc. In particular, when the polyester film is used as a label for a polyethylene bottle, since the polyethylene bottle, as such, is inferior to a PET bottle etc. in heat resistance, the shrinking operation should be conducted at the relatively low temperature of about 70.degree. C. For this reason, the development of a heat shrinkable film having excellent shrinking properties at a low temperature has been desired in the art.
As described in Japanese Unexamined Patent Publication (Kokai) Nos. 63-168329 and 1-136721, a heat shrinkable film comprising a copolymer polyester has been developed wherein an ethylene oxide adduct of a bisphenol compound or an ethylene oxide adduct of a bisphenol derivative is used as a comonomer component and the retention time of residual stress at the time of the heat treatment is prolonged so as to improve the shrinking properties. Even in the heat shrinkable polyester film comprising the above-described copolymer polymer, alleviation in uneven shrinkage is unsatisfactory, and a high percent of shrinkage cannot be attained at a low temperature of about 70.degree. C.
Polyethylene terephthalate, polybutylene terephthalate, etc. have generally been used as a polyester resin for a molding material used in direct blow molding or the like. Further, modified polyester resins comprising said resins copolymerized with various monomers, such as isophthalic acid or cyclohexanedimethanol, polyethylene naphthalate comprising an acid moiety composed mainly of naphthalenedicarboxylic acid, etc. have been developed.
In the modified polyester resins copolymerized with isophthalic acid, however, the mechanical strength properties, such as impact strength, deteriorate with an increase in the amount of isophthalic acid. Further, when direct blow molding is conducted, and since the melt viscosity of the resin is so low that it is very difficult to conduct molding, a comparison cannot be formed, and in particular, a large container cannot be molded. The modified polyester resin copolymerized with cyclohexanedimethanol is unsatisfactory although an improvement in moldability can be attained over that of a resin copolymerized with isophthalic acid. Further, the heat stability of the resin is so poor that when the resin stays within a molding machine for a long period of time, the deterioration of the resin is significant. Polyethylene naphthalate has the feature that it has a higher melting point than the conventional polyester resins, but, it is unsatisfactory as a resin for a molding material with respect to mechanical properties and moldability.
Further, in the polyester resin wherein ethylene glycol is used as a diol moiety, diethylene glycol produced as a by-product is contained in the resin, which leads to a lowering in the moldability and oxidation stability of the resin. An attempt to reduce the diethylene glycol content has been made by the addition of sodium hydroxide or the like during polymerization. Even a polyester resin produced by this method is unsatisfactory with respect to the moldability or oxidation stability although it is possible to reduce the diethylene glycol content to about 1.5% by weight.